Molecular medicine requires agents specifically and efficiently interacting with target cells. The efficient in vivo delivery of functional therapeutic or diagnostic agents to a target tissue or cell still remains one of the biggest obstacles in drug development. One approach is to couple the payloads to a delivery vehicle that specifically targets cells, for example to an antibody. Payloads must be coupled with good stability to assure specific targeting and avoid systemic nonspecific release of the payload. However, to enable entry into the cell, the payload is ideally released at or within target cells. To combine good stability within the circulation with effective release at the target is a major bottleneck in conjugate development. Most state of the art conjugates consist not of one defined molecule type but are a cocktail of molecules with different amounts of payloads coupled at varying positions. A major drawback of these conjugates is that conjugation procedures need to be adapted for each antibody and each payload; therefore payloads cannot easily be interchanged, also the covalent linkage may cause immunogenicity.
A wide variety of recombinant antibody formats have been developed in the recent past, e.g. tetravalent bispecific antibodies by fusion of, e.g., an IgG antibody format and single chain domains (see e.g. Coloma, M. J., et al., Nature Biotech 15 (1997) 159-163; WO 2001/077342; and Morrison, S. L., Nature Biotech 25 (2007) 1233-1234). Also several other new formats wherein the antibody core structure (IgA, IgD, IgE, IgG or IgM) is no longer retained such as dia-, tria- or tetrabodies, minibodies, several single chain formats (scFv, Bis-scFv), which are capable of binding two or more antigens, have been developed (Holliger P, et al, Nature Biotech 23 (2005) 1126-1136; Fischer N., Léger O., Pathobiology 74 (2007) 3-14; Shen J, et al., Journal of Immunological Methods 318 (2007) 65-74; Wu, C. et al., Nature Biotech. 25 (2007) 1290-1297).
Bispecific antibodies are capable of simultaneous binding of two different targets and thus also capable of delivering a great variety of payloads to the target tissues or cells. To date bispecific antibodies specific for certain cell targets have been described. However this methodology has a major disadvantage as it requires that antibodies be raised against every agent desired for diagnostic and therapeutic use.
U.S. Pat. No. 7,429,381 discloses a two-step pretargeting method, wherein a bispecific antibody specific for a HSG hapten and a cell-surface protein is first administered to the cell and is then used to capture a HSG hapten to which a therapeutic or diagnostic cation or a therapeutic or diagnostic agent is chelated to. Haptens are small molecules, such as pesticides, drugs, hormones, and toxins, which are usually nonimmunogenic unless coupled with some macromolecules such as proteins. However the use of bispecific anti-HSG-hapten antibodies in a two-step pretargeting method has several limitations.
The major disadvantage is the complexity of the approach, which involves preparation and dosing of two separate reagents and consequential timing and ratio issues. In addition, it is not possible to analyse the resulting complex of the bispecific antibody and the therapeutic or diagnostic agent. It is therefore difficult to predict pharmacological properties, stoichiometry and possible degradation products of the therapeutic or diagnostic agent captured by the bispecific antibody.
In addition, currently used conjugates can not be applied for most therapeutic agents since conjugation often results in reduced or eliminated activity of the therapeutic agent or in undesired alterations of the binding capabilities of the antibody.
Therefore there is a need for a well-defined, efficient and specific delivery platform for therapeutic and diagnostic agents with effective release of the payload at the target that can be broadly applied.